Gimp stitchpanorama plugin

stitchpanorama is a plugin for the gimp that allows you to merge two photographs together into a partial rectilinear Projection panorama.

You can add control points which are automatically fine-tuned (much like the various GUI front-ends for Panorama Tools). The photos can then be merged into a final output, one image remains unaltered, the other is rotated, blended and distorted to fit (similar to xmerge).